The invention involves methods for melting steel in a closed electric arc resistance furnace in which a layer of slag is used to cover the molten metal. Since the development of a technique for producing iron sponge, it is now possible to produce steel in an electric arc resistance furnace because of the electrical properties of the iron sponge. Due to its content of slag-forming substances and its high porosity, as compared to the previous use of relatively solid charge materials not previously treated to enhance their reducing properties, the iron sponge makes possible a more efficient use of the electrical energy applied to the furnace during the melting process, because of a much better degree of energy distribution.
In steel production today, more and more metallurgical treatments to secure products of specific properties are performed outside the furnace. This is especially true during tapping of the product from the furnace and in the pan receiving the molten steel from the furnace. For example, the pan may be placed under a vacuum in order to obtain certain qualities in the steel product. In some instances, it is appropriate or required to carry out further treatment in additional furnace units, such as an induction channel furnace, for example.
When an electric arc resistance furnace is operated in the usual manner by batch charging and removal of the product, the analysis of the final steel product is subject to certain changes from charge to charge, as changes in the composition of the charge content cannot be avoided. For example, those metallurgical processes which depend to a large extent on the content of carbon, sulphur and phosphorus in the charge, are particularly sensitive to changes in the charge from batch to batch. Thus, with such batch production, the resulting product, which is tapped sequentially in a batched manner or received in a pan, and which is to undergo a subsequent metallurgical refinement, requires additional expensive adjustments in order to compensate for the differences in the different batches as they are received from the furnace.
As will be appreciated with respect to processes as discussed herein, there is a momentary relatively violent interaction between a new charge introduced into the furnace and the heated slag or electrode surfaces with which the new charge comes into contact. This relatively violent interaction causes, as a by-product thereof, the evolution of polluting gases. Because the methods and apparatus herein reduce this momentary violent reaction when a charge is introduced, there is a substantial reduction in gases emitting from the furnace, thus reducing to a substantial amount the polluting gases emanating from the furnace and requirements for equipment to cope with these polluting gases.
With the method of the invention here, the above results are achieved by three basic procedures, including (a) the incorporation into a charge of iron ore a quantity of iron sponge. Alternatively, the ores may be pre-reduced or agglomerated into small pellet-like form or briquets which provide for ready assimilation of the charge into the content being maintained in the furnace chamber, including the slag cover and the molten steel bath. (b) Further, a thick slag cover containing decarbonizing, dephosphorizing and desulphurizing properties is maintained, with the thickness thereof being, preferably, within the range of between about 250-1500 mm. Moreover, the production process is carried on for a much longer period than the previous batch-type processes, and is usually carried on for a period of 1 to 10 days. (c) In addition, a molten metal bath is maintained with its quantity being the same as a quantity which would be produced in a period within the range of 5 hours-2 days.
By utilizing these measures, it is possible to run the electric arc resistance furnace continuously. The iron oxide content in the slag required for decarbonization of the ore into a desired product is achieved by charging iron ore combined with iron sponge. In the alternative, pre-reduced ores or agglomerates may be used, and they are adjusted during the pre-reduction procedures to be of a desired grade. Because of the maintenance of the comparative thick slag layer and the large metal bath, this maintenance serves to balance variations in the properties of the charge introduced from time to time into the furnace chamber. Because of this maintenance of the relatively thick slag layer and large metal bath, newly introduced charge material is rapidly assimilated, thus reducing, as discussed above, the amount of surface area of the charge particles exposed to a violent interaction at the charge-slag interface. This in turn reduces the necessity for varying and controlling temperatures upon the introduction of the charge for the feeding process, as opposed to the subsequent melting process. Then, because of this more uniform application of temperature, there is a lower initial temperature at the charge-slag interface, thus increasing the service life of the furnace lining and the charge inlet surfaces. Moreover, because of this uniformity of energy use, better results are achieved, than with respect to the prior art bath processes with reduced applied energy. Environmental pollution is reduced, as discussed above, because the evaporation of metal at the charge-slag interface is lessened by the rapid assimilation of the charge into the thick slag layer with reduced "hot spots" or burning surfaces.
Of special advantage in the use of the invention here over conventional batch-type charge operations is that for an approximately equal investment cost per ton-year of steel (unrefined) produced, a substantial reduction in the cost of the furnace lining is obtained. The strong thermal stresses on the furnace cover and the side walls thereof are reduced by lessening heat radiation from the furnace electrodes. This is so because the electrodes are either immersed in the thick slag layer or covered by the burden or charge resting on the metal bath. Both because of a reduction in these radiation losses from the electrodes, because they are immersed in the slag during the melting period, and because of reduction in variations in energy requirements during charging operations, the energy requirements per ton of finished product, in accordance herewith, are, conservatively, at least about 10% less than with previously employed methods. Previously, the energy requirements were about 600 kilowatt hours/ton of steel, while with the invention here the energy requirements are about 540 kilowatt hours/ton of steel.
Further with respect to energy requirements, and particularly when electric energy is secured from an outside source, because of the relatively uniform energy requirements over the entire period of operation, there are no peak demands as during charging operations in the batch processes of the past.
With respect to further treatment operations, such as the utilization of strand casting equipment, with the continuous method here, and a continuously tapped and produced product, the strand casting equipment may be operated on a continuous basis, rather than depending upon the intermittent operation of conventional furnaces. Another advantage to the invention here is that because the molten steel bath is covered by the thick slag layer, it absorbs less nitrogen from the air than would be the case without such a thick covering. Furthermore, the method here permits the use of a lesser quality of charging ore because the thick slag layer permits the use of charge materials with a high slag content.
A further teaching of the invention is that it permits the use of hollow electrodes which allow for the simultaneous feed through the electrodes, as the case may be, together with reacting oxygen gas to increase the refining effect. Moreover, in those instances where large masses of steel are to be produced by the method here, the continuous method permits the addition of alloying substances outside the furnace in the pan during tapping, with subsequent alloying aggregate additions or other refinements to the final product.
With the apparatus of the invention here, the furnace chamber is equipped with at least one tap for the molten metal product and one for the slag, which are arranged at different heights from each other. Because of the maintenance of the very thick slag layer and large molten metal bath, a localized cooling may be applied to the slag layer in a concentrated manner.
As illustrative of apparatus which is useful for carrying out the invention, reference is made to the following drawing showing an electric arc resistance furnace.